Within the framework of the present invention, the expression time constraint is understood to mean a constraint which obliges the aircraft to get to a particular flight point at a predetermined time.
Generally, to carry out such a method, the succession of following steps is implemented repeatedly:    a) the actual time at which the aircraft gets to said particular flight point is determined, taking account of the current target speed;    b) a time interval corresponding to the difference between said predetermined time and said actual time is calculated;    c) this time interval is compared with a duration of comparison; and    d) said target speed is updated, which is applied to the aircraft, as soon as said actual time interval becomes greater than said duration of comparison.
Such a standard method of calculation (and of updating) of the target speed generally entails a quasi-permanent updating of said target speed, in particular by virtue of incessant variations of internal and external (to the aircraft) parameters, which are used in a standard manner for this calculation, such as the characteristics of the wind in particular. This is all the more true as the duration of comparison is generally reduced and most often corresponds to a few seconds of flight.
This known method therefore requires an appreciable calculation ability and especially entails a permanent fluctuation of the target speed, this being very penalizing, since:                on the one hand, this fluctuation entails a drop in comfort of the passengers of the aircraft, which is flown at said target speed; and        on the other hand, any fluctuation gives rise to the need to update other prediction parameters which depend on said target speed.        